I see in many string problems like this: Find how many string there is of given length n, consisting only of the characters: 'a', 'b',and "c", and do not contain the substring "ab". In this problem answer of simple tests : n=0 answer=1 why answer is 1??why??
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Empty string is string too?
yes
That question from BekzhanKassenov answers your question. Empty string neither has any invalid character(characters except 'a', 'b', 'c') nor has "ab" as substring. So an empty string is a valid string for this question.
thanks,i understood now.
Be careful of 'empty set answers', and also in other problems, e.g.count how many sets of vertices(or edges)in a tree that satisfy some certain constraints. In many cases empty set will be a correct answer.